Many commercially important metals occur naturally in chemical composition with sulphur and iron, including gold and copper. These sulphidic compounds are difficult to process to a state where the important metals can be recovered.
Methods for separating metals from their sulphidic host minerals fall into two categories: Pyrometallurgical recovery and hydrometallurgical recovery. Pyrometallurgical recovery involves heating the ore mass and in the process decomposing the sulphide through oxidation resulting in the formation of sulphur dioxide gas. Hydrometallurgical recovery on the other hand involves the dissolution of the ore constituents in a liquid medium in which one or more chemical reactions can be initiated which will cause the important metals to form a new, recoverable compound. Pyrometallurgical recovery is unsatisfactory today because of the formation of sulphur dioxide gas in the so-called roaster oxidation reaction. Accordingly, this technique has largely been abandoned due to legislation restricting sulphur dioxide emissions. Hydrometallurgical recovery is also an unsatisfactory process because metal recovery is hindered and, in many cases, rendered practically useless in the presence of sulphidic compounds.
Recent developments in this area include bio-oxidation where bacterial enzymes are used to oxidize sulphidic ores. However, this process is highly sensitive to variables such as temperature, sulphur concentration, and the presence of other minerals that may be toxic to the bacteria. Furthermore, the process is extremely expensive and relatively slow, rendering it commercially unviable in many situations.
Conventional pyrite roaster reactions are described by EQU FeS.sub.2 .fwdarw.FeS+S (I) EQU 4FeS+7O.sub.2 .fwdarw.2Fe.sub.2 O.sub.3 +4SO.sub.2 (II) EQU S+O.sub.2 .fwdarw.SO.sub.2 (III)
In reaction (I), pyrite (FeS.sub.2) is decomposed into pyrrhotite (FeS) and elemental sulphur (S). In the presence of oxygen and at sufficiently high temperature the associated reactions (II) and (III) include the oxidation of pyrrhotite to form hematite and sulphur dioxide, and of sulphur to form sulphur dioxide. These reactions are highly exothermic, hence it is not possible in conventional roasting reactors to prevent the temperature from increasing to the point where SO.sub.2 is produced. In fact, in conventional roaster operation, this exothermic energy is necessary to provide the reaction energy needed to cause (I) to occur. This reaction, when augmented by steam and oxygen, may be used as a means of producing high quality SO.sub.2 as a desired product, as disclosed by Jukkola in U.S. Pat. No. 3,632,312.
An alternative reaction to reactions (I)-(III) is: EQU 2FeS.sub.2 +1.50.sub.2 .fwdarw.Fe.sub.2 O.sub.3 +4S (IV)
by which pyrite is oxidized directly into hematite and elemental sulphur. Table 1 and FIG. 4 present a thermodynamic analysis of this reaction at various temperatures. Tryer, for example, teaches this reaction (IV) in Australian Patent No. 9674. However, Tryer states in his disclosure that it is necessary to maintain the operating temperature within the range 800.degree. C.-1000.degree. C. (1073-1273.degree. K) to promote the combination of a high concentration of SO.sub.2 with ferrous sulphide to produce sulphur, a process which often requires the introduction of additional SO.sub.2 to make up the necessary concentration.
An associated reaction, the combination of sulphur with hematite to form magnetite (Fe.sub.3 O.sub.4) and SO.sub.2 is described by: EQU 3Fe.sub.2 O.sub.3 +S.fwdarw.2Fe.sub.3 O.sub.4 +0.5SO.sub.2 (V)
Table 2 and FIG. 3 present a thermodynamic analysis of this reaction at various temperatures. Referring to Table 2 and FIG. 3 it will be seen that reaction (V) only begins to become significantly favoured over reaction (IV) for temperatures above approximately 800.degree. K (527.degree. C.).
Thus, because of the continuum of reactions, in order for reaction (IV) to be favored and to avoid the entire roaster reaction (I-III) at lower temperatures, the operating temperature must be maintained below approximately 1000.degree. K (727.degree. C.) and, furthermore, in order to avoid the predominance of reaction (V), the temperature must be maintained below approximately 800.degree. K (527.degree. C.). In view of the fact that pyrite reacts in the presence of abundant oxygen according to the reaction described by: EQU 2FeS.sub.2 +5.50.sub.2 .fwdarw.Fe.sub.2 O.sub.3 +4SO.sub.2 (VI)
(shown as the lowermost curve in FIG. 4 from the data in Table 3) and this is the most strongly favoured pyrite reaction, it is also important to restrict the supply of oxygen so that reaction (IV) remains as the favoured reaction to allow for the production of elemental sulphur (central curve of FIG. 4) as opposed to SO.sub.2.
Therefore, to limit SO.sub.2 production, the preferred operating temperature is below 800.degree. K (527.degree. C.), where the reaction products of pyrite and oxygen will be primarily restricted to hematite and sulphur as described in reaction (IV).
The ability to maintain the otherwise highly exothermic oxidation reaction temperature below 800.degree. K (527.degree. C.) requires separate control of: (1) the oxygen supplied to the reaction; (2) the power (energy) introduced into the material; and (3) the gas flow through the reaction environment (coolant). Control of the aforesaid factors can be achieved, in association with the present invention, using a fluidized bed reactor with power supplied by microwave energy, for treating pyritic mineral ore.
Fluidized bed reactors are presently widely used in many ore processing applications where strong interaction between a solid product and gas medium is required and the use of microwave energy to provide some or all of the required reaction energy has been disclosed in, for example, U.S. Pat. Nos. 3,528,179; 4,126,945; 4,967,486; 4,511,362; 4,476,098; 5,382,412 and 5,051,456.
The use of a fluidized bed reactor with a microwave source of power provides the ability to control the oxygen supply to the material undergoing treatment (which governs the rate of reaction and hence reaction temperature) independently of the microwave power (which supplies the energy to initiate the chemical reaction and compensates for other energy losses). The use of microwave energy also provided the unique ability to selectively heat certain materials in the presence of less absorptive gangue materials as is the case with pyritic ores.
The exhaust stream from the reactor is depleted of oxygen as a consequence of the oxidation reaction with the fluidized bed and consists principally of nitrogen. It has been found that by diverting and preferably cooling the exhaust stream and reintroducing it into the reactor with the fluidizing stream that it is effective as a coolant and thus provides the final factor required to achieve the preferred chemical reaction to process pyrite minerals under temperature conditions that provide reaction products that are free of SO.sub.2 emissions.